many years, E.I. du Pont de Nemours and Company (DuPont) has been making TYVEK.RTM. spunbonded olefin. Commercial end uses for TYVEK.RTM. spunbonded olefin sheets have been developed for mailing envelopes, housewrap, apparel, medical packaging and many other uses. The process for making the spunbonded olefin has been the subject of numerous patents including U.S. Pat. No. 3,081,519 to Blades et al., U.S. Pat. No. 3,169,899 to Steuber, U.S. Pat. No. 3,227,794 to Anderson et al., U.S. Pat. No. 3,484,899 to Smith, U.S. Pat. No. 3,497,918 to Pollock et al., U.S. Pat. No. 3,860,369 to Brethauer et al., U.S. 4,352,650 to Marshall, U.S. Pat. No. 4,554,207 to Lee, and U.S. Pat. No. 5,123,983 to Marshall. The basic steps of this process are (1) forming a solution of polyolefin polymer with Freon.RTM. 11 spin agent, and (2) flash spinning the solution in a spin cell. Freon.RTM. is a registered trademark owned by DuPont. However, Freon.RTM. 11 spin agent is a chlorofluorocarbon (CFC) and is believed to be a cause of ozone depletion. The use of most CFC materials are targeted to be eventually banned.
DuPont has sought a substitute spin agent for use in the continued manufacturing of spunbonded olefins. Unfortunately, there is not a readily available spin agent that would be a simple substitute for the Freon.RTM. 11 spin agent. Although it has been found that spunbonded olefins may be made using one of a number of different spin agents, each potential alternative spin agent gives rise to numerous production process or product quality issues. Among the alternative spin agents that have been found for making TYVEK.RTM. spunbonded olefin are certain hydrocarbons, including pentane. An important issue for hydrocarbon spin agents is their flammability whereas Freon.RTM. 11 spin agent is not at all flammable. The issues of flammability and explosivity are substantial when one considers that the spin agents will be subjected to high pressure and high temperature during the flash spinning processes. The solution provided to the spin cell is approximately eighty percent spin agent by weight so the amount of hydrocarbon that may be subjected to the high pressures and temperatures associated with flash spinning is not minimal.
The solutioning system in the process for making spunbonded olefin is the portion of the system that mixes the polymer with the spin agent to form a homogenous solution suitable for spinning into plexifilaments. The solutioning system in current use is generally illustrated in FIG. 1. As illustrated, the system comprises a very large drum 12 arranged to receive measured amounts of polyethylene pellets and spin agent. The polyethylene pellets are supplied from a hopper 14 and the spin agent is supplied from a tank 15. The drum 12 is sized to hold the pellets and spin agent for an extended period of time (e.g. hours) and is approximately 5000 gallons. The drum is closed and maintained at approximately room temperature and pressure. The pellets are rapidly stirred by a rotating agitator 19 to form a uniform slurry. The pellets and spin agent are drawn from the drum 12 into a pressure pump 21 which pumps the polymer slurry so as to raise the slurry pressure while directing the slurry through a heat exchanger 22 to raise the slurry temperature. The high pressure, high temperature slurry is then provided to a dissolver tank 23 where the slurry is stirred and mixed by an agitator 24 until the mixture becomes a homogeneous solution suitable for flash spinning in a spin cell, schematically indicated at 25.
With the conventional solutioning system described above, there had been concern about transient fluctuations in the ratio of spin agent to polymer which may significantly effect the quality and properties of the flash spun product. Thus, there has been considerable effort to blend the solution in the system in a manner that eliminates or substantially reduces such transient fluctuations in the solution. As such, the system provides for large amounts of the solution in both the dissolver tank 23 and the drum 12 at any given time. A plant may spin anywhere from 2000 to 10,000 pounds of polymer per hour and the solution from which this polymer is spun is ordinarily comprised of 75 to 90 percent spin agent by weight. Thus the conventional solutioning process of FIG. 1 requires that tank 23 hold very large amounts of spin agent at high pressure and temperature for extended periods of time. When the non-flammable spin agent is replaced with a highly flammable spin agent, such a large volume of flammable spin agent at high pressure and temperature would raise serious safety concerns.
Accordingly, a solutioning system for a flash spinning process is needed that reduces or avoids the safety concerns described above.
A solutioning system for mixing the polymer with a solvent to form a spin solution is also needed wherein the overall solutioning system has a reduced volume of spin agent as compared to current and conventional solutioning systems.